1. Field of the Invention
The invention relates to a process for preparing pulp of a microfibrous structure suitable for use in manufacturing synthetic paper.
The pulp shall be referred to hereafter for the sake of simplicity as microfibrous pulp.
2. Description of the Prior Art
It is known that in manufacturing conventional paper cellulose pulp is employed. The increasing paper consumption, which grows exceptionally, and the increasing depletion of world forestry supplies, justify the concern that manufacture of cellulose pulp may soon prove insufficient to meet requirements.
For this reason the proposal was made to manufacture paper from synthetic polymeric compounds. To this end a number of processes have been developed for obtaining synthetic materials which have been termed synthetic papers on account of their resemblance to paper both in aspect and printing behaviour.
One of the best known techniques is the manufacture of synthetic papers from film or, more simply, of plastics paper.
By this technique a polymer, which is usually selected from high density polyethylene and polystyrene, is converted to a film, then generally drawn in two orthogonal directions.
In order to make the film similar to paper from cellulose pulp, and suitable for the same use, the starting polymer or the biaxially drawn film are subjected to special treatments.
More particularly, before extrusion the polymer may be admixed with fillers, and above all with pigments such as titanium dioxide, silicon dioxide and calcium carbonate or it may be admixed with swelling agents.
The plastics paper obtained from pigmented film is among the simpler and less expensive ones but suffers from certain not negligible drawbacks, such as the formation of inner weak regions due to unsatisfactory homogenization of the pigment, lower biaxial orientation of the film, unsatisfactory opacity values, unsatisfactory ink receptive properties and attitude towards printing, formation of electrostatic charges during processing. Better results in respect of opacity and printing attitude generally are obtained by admixing the polymer before extrusion with swelling agents which create micropores distributed throughout the thickness of the plastics paper.
The porous structure confers to the plastics paper properties extremely similar to those of cellulose pulp paper but considerably lowers its mechanical properties.
The biaxially drawn film can be made similar to paper by suitable surface treatments (paperization), of a mechanical or chemical nature, or it is coated on both sides with adhesive substances containing opacifying agents.
However, the resulting products are not homogeneous in the direction of thickness, namely they comprise layers of different materials each of which imparts to the product specific properties, the mechanical properties depending upon the intermediate layer, the optical properties and printing attitude depending upon the surface layers. This makes more difficult and delicate any further treatment to which the plastics paper will be subjected, and in particular printing.
Generally, however, as compared with cellulose paper, the plastics paper deriving both from treatment of the polymer and surface treatment of the biaxially drawn film, is of improved properties in respect of water and chemical reagent proofness, tensile strength, dimensional stability and winding attitude.
Among the drawbacks, in addition to those mentioned above, a low tear strength, low folding attitude due to impermeability to air and high manufacturing cost should be mentioned.
On account of these drawbacks plastics paper is employed for special purposes only, above all for uses which take advantage of proofness against both creasing and water and warrant the high costs; especially posters, placards, foldable labels and advertising leaflets.
A further known technique is the manufacture of synthetic paper from continuous filaments or, more simply, "spun bonded" paper.
This article consists of a felt of continuous filaments, the individual fibers of which are glued together at various locations and are uniformly arranged in all directions.
In order to manufacture spun bonded paper the melted polymer is extruded through the orifices of a spinneret in a way similar to conventional processes of manufacturing synthetic fibers.
The thread is continuously drawn then laid on a band where it is submitted to a thermal-mechanical treatment to cause the resulting paper to acquire the desired extent of compactness. The spun bonded paper may further be coated with a suitable lacquer which improves its printing attitude. The important advantage of spun bonded paper is that such papers are obtained by a continuous process with a high production rate. The spun bonded paper is very similar in appearance to a non-woven fabric and has therefore to be submitted to a number of tedious treatments to confer to it an appearance similar to that of cellulose pulp paper.
The cost of manufacture of the spun bonded paper is very high so that the latter also is used only for special purposes such as wall paper, labels and book-covers.
A further known technique is the manufacture of synthetic paper from staple fibers. This method is carried out by either a dry or a wet process. By the dry method staple fibers conventionally prepared from the polymer, a few millimeters or a few centimeters in length, are dispersed for instance by an aerodynamic system. The resulting separate fibers are blown by a gas stream against a paper forming surface to form a sort of felt consisting of variously interlaced fibers.
The resulting material is of low tensile strength thus necessitating a partial adhesion of the individual fibers at their interconnecting and interlocking points by fusing them together. In order to improve adhesion, adhesive substances are also resorted to, which can for instance be spread as latexes on the final article or interlaced with the staple fibers as binding fibers fusible during the end heat treatment.
However, this process is but little used on account of the difficulty of obtaining uniform thicknesses and of the deficient paper properties of the final product in respect to both appearance and mechanical properties.
By the wet method the staple fibers are initially suspended in water and subsequently opened and dispersed by means of the water itself. This step is followed by a process similar to the preparation of cellulose paper from cellulose suspensions.
This method is ditinguished by a high production rate and uniformity in thickness of the resulting product but still suffers from a number of drawbacks.
More particularly, the staple fibers, namely bundles of a number of 10.sup.3 up to 10.sup.6 interconnected fibers should be opened and fibrillated. This usually necessitates a turbulence arrangement. On the contrary, conveying of the suspension of material is advantageously effected by a laminar stream.
Since these requirements can hardly be met, the process should be carried out with a low density of material, namely a small number of fibers per unit of volume water, in order to avoid re-forming of the bundles, which adversely affects the standard of the final product.
Further drawbacks derive from the fact that the cellulose fibers and staple fibers so radically differ from each other that the latter cannot practically be processed by the equipment existing in cellulose pulp paper manufacturing factories.
More particularly, conventional cellulose fibers possess the necessary fundamental properties for making paper sheets, namely they are easily dispersible in water in a uniform manner, are of sufficient length, generally 3 to 6 mm, this length being uniform so that the resulting webs are of a satisfactory strength, and they can moreover be fibrillated and form hydrogen bonds.
On the contrary, synthetic fibers do not possess any of these properties.
The length of synthetic staple fibers generally exceeds 6 mm and the fibers hardly are of a substantially uniform length and most frequently are fused together at their ends.
Finally, since the bundles are deriving from cutting continuous filaments, the so-called tows, generally employed in the textile field, the fibers composing them are usually curled. This all gives rise to heavy drawbacks, as the fibers under these conditions tend to bind together and form entanglements and knots in the finished sheet.
Synthetic fibers are difficult to suspend uniformly in water due to their high water repellent properties so that the suspending medium must be admixed with a surfactant.
Since synthetic fibers can hardly be fibrillated, some steps of the process of manufacture of cellulose paper must necessarily be omitted, more particularly the bending step, for they would degrade the fiber.
Further drawbacks derive from the fact that in conventional paper-making equipment the synthetic fibers tend on account of their above-described properties to become interlaced and form accumulations and obstructions.
For this reason the wet process can hardly be carried out with conventional paper-making equipment for processing cellulose pulp.
The properties of the paper made from fibers by the wet process are not quite satisfactory either, more particularly in respect of tensile strength.
For all these reasons, though the wet process is theoretically of a considerable interest, it could not be widely employed in the paper field, while it was utilized for manufacturing non-woven fabrics used above all in the textile field, such as disposables (handkerchiefs, napkins, disposable articles of wear, pieces of linen), supports for impregnation and coating, felts and the like.
Recently, much work was devoted to the development of synthetic microfibers, representing actual chemical pulps easy to use in paper pulps as a generally partial substitute for cellulose pulp. Processes have been developed which are essentially based on dissolving under pressure polymeric compounds, generally of the polyolefin type, in an organic solvent and on spraying the resulting solution through a nozzle into a medium maintained under conditions of temperature and pressure such as to evaporate the solvent. The result is a synthetic pulp which can be blended with cellulose pulp for the manufacture of paper.
One of the advantages of this technique is that the paper production cycle is not changed as mixtures of cellulose pulp and synthetic pulp do not imply substantial changes in the paper manufacturing line.
The process for the production of microfibrous synthetic pulps as described above, however, suffers from non-negligible drawbacks. Firstly, the cost is very high on account of the restricted possibility for selecting a suitable solvent, the large quantities of solvent required, hence the cumbersome recovery steps and care required by the various processing steps. However, the main drawback resides above all in the fact that the use of microfibrous pulps prepared by such process is not at all simple in the manufacture of paper. For this reason, for instance, synthetic pulps obtained by employing an organic solvent cannot fully replace cellulose pulp, but are always utilized blended with the latter, usually in very small proportions. This is probably all imputable to the fact that the microfibers composing the synthetic pulp obtained when using an organic solvent do not possess hydrophilic properties and can therefore hardly be put in suspension in water, are of low homogeniety and highly differ in structure from cellulose microfibers so that they are ultimately hardly compatible with the latter. Finally, the product obtained by spraying is frequently in the form of a fibrous mass of a continuous structure, which cannot be disaggregated by conventional means into elementary microfibers and is highly swollen by the solvent, its properties being therefore such that it cannot be converted to sheets by conventional paper-making techniques.
Processes were further proposed, which are essentially based on the initial preparation of aqueous emulsions of solutions of polymeric compounds which are crystalline at high temperature and pressure and subsequent spraying of the emulsions in a medium at lower temperature and pressure.
This results in crystalline microfibers, which therefore a high extent of molecular orientation, and highly fibrillated.
As compared with the products obtained by employing an organic solvent, these microfibers are distinguished by a by considerable improvement in their capacity for being put in suspension in water, compatibility with cellulose pulp and possibility of use in the paper field by conventional techniques.
It is therefore believed that a highly important condition for using synthetic microfiber pulps in the paper-making field is to provide microfibers of a very high orientation extent. The latter technique, however, also suffers from not negligible drawbacks. More particularly, spraying does not directly yield elementary microfibers, as required for use in the paper-making field, but rather a fibrous aggregate which cannot be directly employed in preparing paper without being disaggregated into its elementary microfibers; to this end, however, tedious mechanical operations, which are not easy to accomplish, are necessary.
Moreover, these microfibrous pulps, though giving better results in the paper field than the microfibrous pulps obtained with the use of an organic solvent, are employed for the purpose mostly only in a blend with cellulosic pulp which is in any case still the component present in a larger proportion in the blend.
Finally, paper sheets obtained by utilizing synthetic pulp alone, prepared by aqueous emulsion without any addition of cellulosic pulp, are of a very low consistency and require for use further treatments which would make the process cumbersome.